This invention relates to the manufacture of electromagnetic components from amorphous metal ribbons by compressing and bonding said ribbons.
Electrical steel forms the magnetic core of almost all transformers, generators and motors. The machines in which they are employed are usually large and heavy, so that the cost per pound of magnetic material is important. Accordingly, their cores are made of electrical steel because it is the cheapest magnetic material, albeit far from the most effective. For example, the resistivity for grain-oriented silicon steel of 12-15 mil gauge is .about.50, and .about.15 for low carbon steel as opposed to .about.150.mu. .OMEGA.cm for amorphous magnetic alloys.
Cores are subjected to alternating and/or rotating magnetic fields and because the machine in which they are employed handle large amounts of electric power, the minimization of the energy loss per cycle is quite important. The losses are primarily due to eddy currents. Eddy currents are objectionable, not only because they decrease the flux, but also because they produce heat. These currents which oppose the main field can be decreased by forming the core of thin sheets rather than from a solid piece. If the sheets are electrically insulated from one another, the eddy currents are forced to circulate within each lamination. Not only is the path length in each lamination now shorter but the cross-sectional area of the path is also reduced. The induced emf is therefore reduced and the net effect is a decrease in the current and in the eddy-current power loss. For these reasons, laminated construction is standard for all cores of transformers, motors or generators made from metallic conducting materials.
In order to minimize the cost of construction, the laminations are usually thicker than would be desired to minimize eddy current loss. For example, the most popular lamination thickness is about 0.012 inch, whereas for many applications laminations of 1-2 mils would be desirable. Due to the cost of forming thin sheets of electric steel and the concomitant difficulty and the cost of forming the resultant core, it would be desirable if cores could be made from new materials which have fabrication costs of thick laminations but the magnetic and electrical properties of thin laminations. It is the provision of such magnetic components to which this invention is directed.
Amorphous magnetic metals, unlike normal crystalline magnetic metals, have no long range atomic order in their structure. Therefore, the directionality of properties such as magnetization normally associated with crystal anisotropy is absent. Also, unlike normal metals, amorphous metals are extremely homogeneous, being devoid of inclusions and structural defects. These two characteristics--magnetic isotropy and structural homogeneity--give amorphous metals unusually good d-c magnetic properties. The magnetic isotropy leads to extremely low field requirements for saturation, and the structural homogeneity allows the magnetization to reverse with extremely low fields (i.e., a low coercive force). These two features combined with the high resistivity (15 times that of common iron) and lamination thinness provide a material with the lowest a-c losses of any known high magnetic saturation material.
Amorphous structures can be obtained by several techniques. Electroplating, vapor deposition, and sputtering are all techniques where the material is deposited on an atom by atom basis. Under specific conditions, the atoms are frozen in place on contact and do not have a chance to move to the lower energy positions of the normal crystal lattice sites. The resulting structure is an amorphous, non-crystalline glassy one. These methods, however, are not economical for producing large commercial quantities.
The other method for producing amorphous structures in metals is by cooling rapidly from the liquid melt. Two conditions must be met to achieve the amorphous structure by this method. First, the composition must be selected to have a high glass transition temperature, T.sub.g, and a low melting temperature, T.sub.m. Specifically, the T.sub.g /T.sub.m ratio should be as large as possible. Second, the liquid must be cooled as rapidly as possible from above T.sub.m to below the T.sub.g. In practice, it is found that to produce metallic glasses, the cooling rate must be of the order of a million degrees centigrade per second. Even at these high rates, only special compositions can be made amorphous. Typically, "glass forming" atoms such as the metalloids, phosphorus, boron, silicon, and carbon are required additions to the metal alloy, usually in the 10 to 25 atomic percent range.
In machines, such as motors and transformers, there are design requirements on the geometry of the magnetic material. These requirements depend on the properties of the material and the physical structure of the device. Ideally, the material should be continuous along the flux path to form a completely closed magnetic circuit. This would provide the highest permeability possible for the circuit and the lowest excitation current requirements. This geometry is not possible with normal laminated electrical steel because the assembly requirements necessitate cutting the magnetic material. For example, in transformers the negative effect on the permeability from this cutting is partially eliminated by making a complex interleaved joint; while in motors a substantial air gap remains in the magnetic circuit at the interface between the rotor and stator. Another special geometric requirement on an a-c machine is that the magnetic material be thin in a plane parallel to the flux direction. This is essential to minimize the eddy current losses. However, with decreasing lamination thickness, more laminations are needed so the punching time and assembly costs increase.